Polyetherimides constitute a class of polymers which are useful for a variety of purposes, such as in coatings, films and molded articles. Typically, these copolymers are produced by the reaction between a diamine and one or more dianhydrides, either in the presence of a solvent or by melt polymerization.
In a process described in U.S. Pat. No. 3,917,643 (Takekoshi, et al.), a polyetherimide is prepared by the reaction of an aromatic bis(ether anhydride) and a diamine in a phenolic solvent. The result is a solution of the polymer in the phenol which can be used directly as a wire coating enamel, or, alternatively, which can be treated to separate the polymer.
A process for the preparation of a copolyetherimide from a mixture of an organic diamine, a bisphenol dianhydride and a comonomeric dianhydride, for example, pyromellitic, sulfur or benzophenone dianhydride, is described in U.S. Pat. No. 3,983,093 (Williams, et al.). The procedure is based on the use of reaction temperatures of from about 100.degree. to about 250.degree. C. if solvent polymerization is employed, and 200.degree. to 400.degree. C. if melt polymerization is utilized. No criticality as to the order of addition of the coreactants is disclosed, and as shown in the examples all of the ingredients are included in the reaction mixture before polymerization is begun.
Experiments have shown that if the dianhydrides and diamine are added to the initial reaction mixture and polymerization is then initiated and run to completion, the resulting copolyetherimide is not completely transparent upon molding, particularly if it contains more than 5 mole percent of the comonomer dianhydride. The reduced optical clarity can be disadvantageous if the copolymer is intended for industrial and commercial purposes where full transparency is an important feature.